Conventionally, when a mold is produced from green sand, the green sand is dropped into a flask by means of gravity and then the sand is squeezed. Alternatively the green sand is filled with compressed air at around 0.3 MPa and then the sand is squeezed. These are the common methods (for example, see Japanese Patent Laid-open Publication No. H11-277183).
In a manufacturing site, if a casting that has been cast by using green sand has a rough surface on it, a surface hardener is sprayed on it or the density of the green sand is increased by increasing the pressure for squeezing.
In a foundry that uses green sand, if the surface of a mold is brittle, a part of the sand drops from the surface when pouring molten metal. That part of the sand flows with the molten metal. Thus that causes a bad casting. To prevent green sand from becoming brittle after molding, a proper amount of moisture, bentonite, etc., are added to the green sand.
After the green sand is filled into a flask by gravity or aeration of compressed air, it is mechanically squeezed and compacted to a predetermined density. If it is filled into the flask by gravity, there may be variations in the density in some small areas. Further, the sand cannot be sufficiently filled into sections that have small diameters. If the green sand is filled by the aeration of compressed air, the moisture in it may be reduced while being filled, because air with a high pressure, such as 0.3 MPa, is used. Further, fine particles of bentonite may be blown out of the sand by the compressed air at the high pressure. Thus the bonding force between the particles of the sand may deteriorate. Therefore a sufficient strength may not be generated on the surface of the mold by squeezing. A mold that is brittle as discussed above may cause a bad casting.
Thus it is the object of the present invention to provide a method for green sand molding that prevents a bad casting by producing a mold that does not have a brittle surface.